WASHINGTON (AFP) - The American soldier released in a swap with the Taleban had been discharged from the US Coast Guard before he joined the army because he could not adapt to military life, officials said on Wednesday, June 11, 2014.

Even Sergeant Bowe Bergdahl's closest friends were dismayed when they learnt he had signed up for the army after his abbreviated stint in the US Coast Guard in 2006, The Washington Post reported.

Sgt Bergdahl went missing in 2009 during his deployment with US troops in eastern Afghanistan and was held by insurgents for nearly five years until he was released on May 31 in an exchange for five Taleban militants held at Guantanamo.

The circumstances around his disappearance in eastern Afghanistan have prompted speculation that he deserted his post.

Before joining the army, Sgt Bergdahl enlisted in the US Coast Guard in 2006, but he did not make it beyond basic training, officials said.

A defence official said he was discharged from the Coast Guard after only 26 days.

His departure was labelled as an "uncharacterised discharge" and the reason cited was a "failure to adapt to military life", the defence official told AFP on condition of anonymity.

Returning from his time in the Coast Guard, Sgt Bergdahl told friends he had faked a psychological disorder to get out, according to the Post.

His friends were sceptical of his account and two years later when he enlisted in the US Army, they were shocked by his decision and the fact that he was accepted.

"I was like, 'Why and how did you even get in?'" his close friend Kim Harrison said. "'How did they let you?' I was furious."

A journal and other writings obtained by the Post that date back to the months before he disappeared convey a troubled young man struggling to maintain his mental stability.

"I am the lone wolf of deadly nothingness," Sgt Bergdahl wrote in one passage quoted by the Post.

After he went missing in 2009, Sgt Bergdhal's journal and other personal items were sent to Ms Harrison, who he had designated as the person who should receive his remains if he was killed.

Ms Harrinson asked the Post to be identified by her former married name because she was concerned about threats.

Sgt Bergdahl's friend said she shared the journal and other correspondence with the newspaper because she had become concerned that the soldier was being falsely portrayed as a calculating deserter.

Instead, she described him as a sensitive, vulnerable young man who was simply not cut out for the regimented life of the military.

"He is the perfect example of a person who should not have gone" to war, Ms Harrison said.